WO2002103341A1

WO2002103341A1 - Conduvtivity sensor

Info

Publication number: WO2002103341A1
Application number: PCT/TR2002/000022
Authority: WO
Inventors: Ibrahim Tanriverdi; Nazmi ÖZTÜRK
Original assignee: Arçelik A.S.
Priority date: 2001-06-18
Filing date: 2002-06-18
Publication date: 2002-12-27

Abstract

The conductivity sensor (1) comprises a sensor circuit (2), an amplifier circuit (3), a rectifier circuit (4), a peak detector (5) and a reset circuit (6). The sensor circuit (2) comprises an operational amplifier (7), two electrodes (9) and a resistance (8). By means of this circuit, the current passing between two electrodes (9) under steady voltage, is evaluated instead of the voltage value falling on two electrodes (9). The problems of polarization, physical and chemical resistance are eliminated by using graphite electrodes (9).

Description

CONDUCTIVITY SENSOR

The present invention relates to a conductivity sensor that is placed in the washing machine tub to provide the measurement of the conductivity of the washing solution.

In the current applications, water — hardness and the time and degree of dissolution of the detergent disposed in the machine are determined by means of the sensors placed in the washing machine tub, and thereby the washing programme can be controlled.

In the state of art, in the PCT Patent Application No. 9512704, developed for measuring the conductivity of the washing solution, a control unit is used and the established solution conductivity values are stored in its memory. The washing machine takes in water and an amount of detergent, after a mixing time of less than two minutes the conductivity of the wash liquor is measured and said measured conductivity value and the values in the memory are compared in order to decide whether or not to add more detergent to the washing solution. However, this Patent Application does not contain any information related to determining the water intake amount and the duration of rinsing.

In the European Patent Application No. 0383218, the conductivity value of the water taken in the machine is measured by a sensor located at a point near the water intake and stored in the memory. The conductivity of the residual water - from the rinsing steps, in the discharge hose, is measured by a sensor and is compared with a reference value determined by the manufacturer, so that a decision can be made as to whether a further water intake is necessary.

The conductivity of a solution is found by measuring its resistance against the electrical current. The conductivity of the solutions depends on the amount of electrolytic material contained and the temperature of the solution. The anionic surfactant material present in the detergent, increases the conductivity of water. Furthermore, detergent comprises a great amount of inorganic substances (phosphate, silicate, carbonate) and the contribution of these substances to the conductivity of the washing solution is much more than that of the other compounds of the detergent. For this reason, information related to the detergent distribution can be obtained by deteimining the distribution of conductivity in the washing medium.

The detergent distribution in the machine, may be followed by the conductivity measuring devices. However, a great number of measuring devices are required to be employed in order to take measurements at several points in the machine. The most important disadvantage of the conductivity measuring devices is that they are not able to take continuous (uninterrupted) measurements and that their memory capacities are limited.

In the state of art, sensors with four electrodes are used for the measurement of solution conductivity. In such practices, an alternating current is applied through external electrodes with predetermined section area and length. Whereas the electrodes located at the inner sides, detect the electrical area (stray field) formed by the current passing through the solutio and obtain information to provide the conductivity value of the solution. As the measurement information is directly derived from the solution medium, this system is not suitable for multiple applications. The voltage differences between electrodes of different sensors may cause cross current passages between said electrodes which have a detrimental effect on the accuracy of the measurement value with an indefinite and variable ratio.

In another embodiment of the state of art, alternating current is applied on the electrodes in the sensor over a resistance and the voltage on the electrode is measured. As the voltage falling on the electrodes change with conductivity, cross current may be formed in multiple applications. Thus, the measurements made under this condition are not reliable. Since the electrodes are completely immersed in water, the conductivity of the environment may vary due to the mobility (during rinsing, draining, etc.) of the environment and as the result of the decrease in conductivity in connection with said mobility, the voltage on the sensor rises and when this voltage value exceeds a certain threshold value, electrolysis occurs. As the result of this chemical reaction, the sensor is deformed and erroneous information is obtained. Moreover the voltage values falling on each sensor are different from each other; this difference thereby results in the formation of unwanted cross currents between the sensors, which in turn leads to unreliable measurements. Square waves are applied on the sensors and measurements are taken after the voltage surpasses the voltage peak value and comes back to a steady state. In the conductivity measuring sensor with two electrodes, the steady state electrode voltage or the actual^' (current) electrode current does not exhibit an acceptable relationship with the solution concentration. Furthermore, the fact that the measurement environment is in a continuously mobile state, prevents the formation of a healthy steady (stable) status.

The object of the present invention is to realize a conductivity sensor to be used in washing machines, the electrodes of which are resistant against polarization and corrosion, and which is not effected by the heat and washing material in the operational environment.

The conductivity sensor realized in order to attain the above object of the present invention has been illustrated in the attached drawings, wherein;

Figure 1, is the schematic view of the conductivity sensor circuit, Figure 2, is the schematic view of the sensor circuit.

The components shown in the drawings have been enumerated separately as follows : 1. Conductivity sensor

2. Sensor circuit

3. Amplifier circuit

4. Rectifier circuit 5. Peak detector

6. Reset circuit

7. Operational amplifier

8. Resistance

9. Electrode

The conductivity sensor '(1) comprises a sensor- circuit (2), an amplifier circuit (3), a rectifier circuit (4), a peak detector (5) and a reset circuit (6). (Fig. 1).

The sensor circuit (2) comprises an operational amplifier (7), two electrodes (9) and a resistance (8). By means of this circuit, the current passing between two electrodes (9) under steady voltage, is evaluated, instead of the voltage value falling on two electrodes (9). (Fig.2).

The operational amplifier (7) used in the sensor circuit (2) is a circuit element with high gain, that uses voltage feed back in order to provide a steady voltage gain. Said operational amplifier (7) provides high gain when no feedback signal is present, with high input and low output impedance. The inlets (ports) of the operational amplifier (7) are shown as plus (+) and minus (-) ports in order to specify the irreversible and reversible inputs. While the signal applied on the plus port is seen as having the same polarity and is amplified at the outlet, the signal applied on the minus port is seen as amplified but reversed, at the outlet.

In the conductivity sensor (1), the input signal is applied on the plus (+) inlet

(port) of the operational amplifier (7), and the output of the operational amplifier (7) is connected to the minus (-) port by R_ref resistance. One of the electrodes (9) is connected to the minus (-) inlet (port) of the operational amplifier (7) and the other electrode is connected to ground. In this way, by providing the functioning of the operational amplifier (7) as a regulator, the formation of the same V] voltage that is applied to the inlet port of the operational amplifier (7) and which has a square wave signal being symmetrical with regard to the ground, over the electrodes (9) is provided. During the measurement, the same current (I_sensor) passing through the sensors also passes over R_ref. Thereby, as the solution conductivity varies, the current (Ise_ns_or) passing between the electrodes, and in connection with it, the current passing over R_ref, also change. The voltage value (V₂) formed by the current (I_senso_r) passing over R_ref, on the R_ref resistance is taken as the measurement information. The voltage (V₂) obtained over R_ref, is amplified by an amplifier circuit (3) that operates as a differential amplifier, is rectified by a rectifier circuit (4) that is sensitive to small signals, and is brought to a direct, rectified voltage status by the aid of a peak detector (5) whielf^'is connected to said rectifier circuit (4). By using the resetting circuit (6) the corrected measurement information is brought into a usable condition.

During the measurement, symmetrical square wave is applied around the zero axis, to the electrodes (9) and it is seen that the peak current value obtained immediately thereafter is in a correlation with the solution concentration. By applying voltages of various ratios on the electrodes (9) and by making suitable modifications in electronic circuit parameters, a relatively smooth peak current / solution concentration correlation, in the desired operational range is obtained.

The electrodes are placed in the laundry or dish washing machine so that they are totally immersed in water, and the measurement values obtained from said electrodes (9) are processed by the control unit, whereby the optimization of the rinsing duration, number of rinsing steps and the amount of the water used in laundry or dish washing machines, is provided.

When more than one conductivity sensor is intended to be used the same potential (voltage), value is simultaneously applied on the electrodes (9) located on all sensors and thus the problem of cross-current is eliminated. Furthermore, such problems as polarization, physical and chemical resistance, etc. are eliminated by using graphite electrodes (9).

Claims

1. A conductivity sensor (1) comprising a sensor circuit (2) that includes an operational amplifier (7), two electrodes (9) and a resistance (8), wherein the input signal is applied on the plus (+) inlet (port) of the operational amplifier

(7), the output of the operational amplifier (7) is connected to the minus(-) port by R_ref resistance, one of the electrodes (9) is connected to the minus (-) inlet (port) of the operational amplifier (7) and the other electrode (9) is connected to ground, thus, the formation of the same Vi voltage that is applied to the inlet port of the operational amplifier (7) and which has a square wave signal being symmetrical with regard to the ground, is provided over the electrodes and as the same current (Isensor) passing over the electrodes also passes over R_ref. ; thereby, as the solution conductivity ^"varies, the current (I_sens_or) passing between the electrodes, and in connection with it, the current passing over R_ref, also change during measurement and the voltage value (V₂) formed by the current (I_sens_or) passing over R_ref5 on the R_ref resistance is taken as the measurement information.

2. A conductivity sensor (1) as defined in Claim 1, comprising a sensor circuit (2), an amplifier circuit (3) providing the amplification of the voltage (V₂) obtained over R_ref, a rectifier circuit (4) that is sensitive to small signals and which rectifies said signals, a peak detector (5) which is connected to said rectifier circuit (4) and a reset circuit (6) used to eliminate the faults/errors.

3. A conductivity sensor (1) as defined in Claims 1 and 2, characterized with the electrodes (9) made of graphite material in order to eliminate the problems of polarization, physical and chemical resistance